[BUGFIX release] Add helpful error for {{each}} with duplicate keys.

packages/ember-htmlbars/lib/helpers/each.js

@@ -1,4 +1,5 @@
 import Ember from 'ember-metal/core';
+import Error from 'ember-metal/error';
 import normalizeSelf from 'ember-htmlbars/utils/normalize-self';
 import shouldDisplay from 'ember-views/streams/should_display';
 import decodeEachKey from 'ember-htmlbars/utils/decode-each-key';
@@ -79,13 +80,19 @@ export default function eachHelper(params, hash, blocks) {
   }
 
   if (shouldDisplay(list)) {
+    let seenKeys = {};
     forEach(list, (item, i) => {
       var self;
       if (blocks.template.arity === 0) {
         self = normalizeSelf(item);
       }
 
       var key = decodeEachKey(item, keyPath, i);
+      if (seenKeys[key] === true) {
+        throw new Error(`Duplicate key found ('${key}') for '{{each}}' helper, please use a unique key or switch to '{{#each model key="@index"}}{{/each}}'.`);
+      } else {
+        seenKeys[key] = true;
+      }
       blocks.template.yieldItem(key, [item, i], self);
     });
   } else if (blocks.inverse.yield) {

packages/ember-htmlbars/tests/helpers/each_test.js

@@ -1302,5 +1302,39 @@ QUnit.test('can specify `@identity` to represent mixed object and primitive item
   equal(view.$().text(), 'foobarbaz');
 });
 
+QUnit.test('duplicate keys trigger a useful error (temporary until we can deal with this properly in HTMLBars)', function() {
+  runDestroy(view);
+  view = EmberView.create({
+    items: ['a', 'a', 'a'],
+    template: compile('{{#each view.items as |item|}}{{item}}{{/each}}')
+  });
+
+  throws(
+    function() {
+      runAppend(view);
+    },
+    `Duplicate key found ('a') for '{{each}}' helper, please use a unique key or switch to '{{#each model key="@index"}}{{/each}}'.`
+  );
+});
+
+QUnit.test('pushing a new duplicate key will trigger a useful error (temporary until we can deal with this properly in HTMLBars)', function() {
+  runDestroy(view);
+  view = EmberView.create({
+    items: A(['a', 'b', 'c']),
+    template: compile('{{#each view.items as |item|}}{{item}}{{/each}}')
+  });
+
+  runAppend(view);
+
+  throws(
+    function() {
+      run(function() {
+        view.get('items').pushObject('a');
+      });
+    },
+    `Duplicate key found ('a') for '{{each}}' helper, please use a unique key or switch to '{{#each model key="@index"}}{{/each}}'.`
+  );
+});
+
 testEachWithItem('{{#each foo in bar}}', false);
 testEachWithItem('{{#each bar as |foo|}}', true);